[PATCH 3/3] hugetlb: before freeing hugetlb page set dtor to appropriate value

[Mike Kravetz]

When removing a hugetlb page from the pool the ref count is set to
one (as the free page has no ref count) and compound page destructor
is set to NULL_COMPOUND_DTOR.  Since a subsequent call to free the
hugetlb page will call __free_pages for non-gigantic pages and
free_gigantic_page for gigantic pages the destructor is not used.

However, consider the following race with code taking a speculative
reference on the page:

Thread 0				Thread 1
--------				--------
remove_hugetlb_page
  set_page_refcounted(page);
  set_compound_page_dtor(page,
           NULL_COMPOUND_DTOR);
					get_page_unless_zero(page)
__update_and_free_page
  __free_pages(page,
           huge_page_order(h));

		/* Note that __free_pages() will simply drop
		   the reference to the page. */

					put_page(page)
					  __put_compound_page()
					    destroy_compound_page
					      NULL_COMPOUND_DTOR
						BUG: kernel NULL pointer
						dereference, address:
						0000000000000000

To address this race, set the dtor to the normal compound page dtor
for non-gigantic pages.  The dtor for gigantic pages does not matter
as gigantic pages are changed from a compound page to 'just a group of
pages' before freeing.  Hence, the destructor is not used.

Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
---
 mm/hugetlb.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3132c7395743..fa8ec2072949 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1370,8 +1370,28 @@ static void remove_hugetlb_page(struct hstate *h, struct page *page,
 		h->surplus_huge_pages_node[nid]--;
 	}
 
+	/*
+	 * Very subtle
+	 *
+	 * For non-gigantic pages set the destructor to the normal compound
+	 * page dtor.  This is needed in case someone takes an additional
+	 * temporary ref to the page, and freeing is delayed until they drop
+	 * their reference.
+	 *
+	 * For gigantic pages set the destructor to the null dtor.  This
+	 * destructor will never be called.  Before freeing the gigantic
+	 * page destroy_compound_gigantic_page will turn the compound page
+	 * into a simple group of pages.  After this the destructor does not
+	 * apply.
+	 *
+	 * This handles the case where more than one ref is held when and
+	 * after update_and_free_page is called.
+	 */
 	set_page_refcounted(page);
-	set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
+	if (hstate_is_gigantic(h))
+		set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
+	else
+		set_compound_page_dtor(page, COMPOUND_PAGE_DTOR);
 
 	h->nr_huge_pages--;
 	h->nr_huge_pages_node[nid]--;
-- 
2.31.1